As known, the most common ski mountaineering boots substantially consist of a shell made of rigid plastic material which is shaped so as to accommodate the user's foot, and is provided on the bottom with a front sole and a rear heel, usually provided with a lugged profile and made of a non-slip elastomeric material; with a cuff made of a rigid plastic material, which is C-shaped so as to envelop the user's ankle from behind, and is hinged to the upper part of the shell so as to oscillate about a transversal reference axis substantially coinciding with the articulation axis of the ankle; with an inner shoe made of soft, heat-insulating material, which is removably inserted into the shell and the cuff, and is shaped so as to envelop and protect both the foot and the lower part of the user's leg; and with a series of manually-operated closing hooks, which are appropriately distributed on the shell and on the cuff, and are structured so as to tighten the shell and the cuff in order to immobilize the user's leg inside the shoe.
The shell of ski mountaineering boots is usually provided on the front with a small, substantially duck-billed projecting appendix, which protrudes from the nose-shaped tip of the shell remaining locally substantially coplanar with the front sole, and is structured so as to be coupled in rigid, stable, although easily releasable manner, with the toepiece of the ski mountaineering binding device which, in turn, is rigidly fixed onto the central part of the downhill ski.
The ski mountaineering binding device instead consists of a toepiece and a heelpiece, which are rigidly and stably fixed to the back of the downhill ski, at a predetermined distance from each other, and are structured so as to alternatively and as desired:                lock the shell of the ski boot onto the back of the ski, thus preventing any relative movement between the two elements; or        lock the shell of the ski boot onto the back of the ski thus allowing the boot to freely oscillate/pivot with respect to the ski about a transversal rotation axis arranged horizontally and roughly positioned at the duck-billed appendix of the shell.        
Obviously, the rotation axis of the ski boot is perpendicular to the rotation axis of the downhill ski, i.e. is oriented so as to be locally substantially perpendicular both to the middle plane of the ski and to the middle plane of the ski boot.
In particular, the toepiece is usually provided with a gripper-like clamping member, which is structured so as to clamp and stably retain the projecting duck-billed appendix of the shell, while allowing the shell to freely oscillate/pivot with respect to the ski underneath about the rotation axis of the boot.
The heelpiece of the binding device, instead, is structured so as to selectively hook and lock the rear part of the shell, so as to selectively prevent the boot from rotating by pivoting on the toepiece and moving the heel away from the back of the ski.
More in detail, the heelpiece is usually provided with a pair of projecting pins which jut out from the turret towards the toepiece, next to each other, from opposite sides of the middle plane of the turret, while remaining locally substantially parallel to a reference axis which is locally substantially parallel to the longitudinal axis of the ski. The ends of the two projecting pins are structured so as to engage the rear part of the shell, roughly at the heel, so as to stably hold the heel of the ski boot in abutment on, or however close to, the back of the ski, thus preventing the ski boot from rotating on the toepiece.
In order to allow the automatic unlocking of the binding device if the skier falls, the two projecting pins are structured so as to be elastically spread, in the presence of particularly strong pulse-like mechanical stresses, elastically by a few degrees with respect to each other, while always remaining on a horizontal laying plane locally perpendicular to the middle plane of the turret.
Unfortunately, the above-described automatic unlocking system is not very sensitive to pulse-like mechanical stresses with an inclination angle larger than 10-15° with respect to the vertical.